Pour boiling water on gelatin; stir until dissolved. Refrigerate until thickened, but not set. Add enough cold water to lemonade concentrate to measure 2 cups; stir into gelatin. Beat until foamy. Beat whipping cream in bowl until stiff; fold into gelatin mixture.
Tear cake into 1 inch pieces. Fold in gelatin mixture; spread in ungreased 13 x 9 x 2 rectangular baking dish; sprinkle with coconut. Refrigerate until firm. Cut into squares.